What happened
On November 19, 2011, an ATR 42-3-20A, registration PP-PTD, was conducting a local flight test near Belo Horizonte, Brazil. The flight was intended to verify the functionality of the aircraft's stall protection systems as part of a delivery process to a new operator. The crew, which included two pilots and four maintenance technicians, was operating at 15,000 feet.
During the execution of a low-speed test (Stall Clean Configuration), the aircraft entered an abnormal attitude. The crew's control inputs at low speeds induced an excessive angle of attack on the left wing, leading to a stall and a subsequent left-hand spin. The aircraft lost approximately 9,300 feet of altitude before the crew was able to recover control. Following the recovery, the aircraft returned to Aeródromo Pampulha and landed without further incident. There were no injuries to the six occupants and no damage to the aircraft.
The investigation
CENIPA's investigation focused on the technical failures and operational procedures during the flight test. Investigators examined the aircraft's maintenance history, noting that the stall warning system tests were being performed in flight despite the manufacturer's recommendation to conduct such checks on the ground using jacks.
The investigation also reviewed the crew's training and the company's flight check documentation. It was found that while the pilots were qualified to operate the type, they had not undergone formal theoretical or practical instruction specifically for performing flight checks on the ATR 42-320A. Furthermore, the flight check sheet used by the operator contained errors, specifically a lack of calculated stall speeds and missing warnings regarding the failure of the stick shaker and pusher systems.
Findings
- Improper control inputs: The sequence of control inputs applied at low speeds caused the left wing to exceed the maximum angle of attack, triggering the stall and spin.
- Inadequate training: The flight test exceeded the crew's demonstrated capability, and the procedures for recovery after stall or uncommanded roll control, as outlined in the Flight Crew Operating Manual (FCOM), were not correctly applied.
- Maintenance and testing risks: The decision to perform a full stall in flight was unnecessary, as the manufacturer provided procedures for verifying the stall warning system on the ground using jacks.
- Defective documentation: The company's flight check sheet contained printing errors and failed to provide critical speed parameters or warnings for system failures.
- Management and supervision: There was a lack of adequate oversight regarding crew qualifications for test flights and a failure to follow the manufacturer's recommended testing protocols.